One form of drive roller unit for driving an article on a conveyor track, as disclosed in European patent application No. 89 102 955.5, corresponding to U.S. application USSN 377,971, now U.S. Pat. No. 4,949,837, comprises two drive rollers which can be driven in rotation by an electric motor means by way of a transmission means and which in their drive position are in engagement with the underside of the article to be driven. The two drive rollers are arranged with their axes of rotation substantially transversely with respect to the direction of drive of the article and in juxtaposed relationship in their axial direction, while the transmission means includes a first transmission wheel which is driven by the electric motor means and which drives in rotation the first drive roller, and a second transmission wheel which is coupled to the first transmission wheel and which drives in rotation the second drive roller. The drive rollers are mounted in a holding means which is mounted rotatably about a vertical axis with respect to a base structure of the conveyor track. A drive roller unit of that kind is used for example for driving freight containers which are movable on roller-type and ball-type conveyor tracks, a preferred area of use of such arrangements involving freight loading systems in air freight traffic, in which the containers are transported in an aircraft freight compartment on roller-type or ball-type conveyor tracks, to the intended anchorage locations.
A drive roller unit of simpler construction for use in such a situation for that purpose is to be found for example from U.S. Pat. No. 4,792,037, comprising a single drive roller which is firstly lifted out of a lower rest position by an electric motor means until it reaches a raised operative position in which it bears against the bottom of the article to be driven along on the conveyor track. In that raised position the drive roller is then driven in rotation by the same electric motor means so that it applies to the underside of the article a tangential force which displaces it in a direction substantially normal to the direction of the axis of rotation of the drive roller.
When loading freight containers into aircraft freight compartments, a problem which now arises is that the elongate containers which are frequently longer than the width of the freight compartment of the aircraft must be pushed into the freight compartment with one of their narrow sides leading, through a cargo hatch in the side of the aircraft, and then turned through 90.degree. so that they can be moved along with their longitudinal axis parallel to the longitudinal axis of the aircraft, until they reach the intended anchorage locations. For the purposes of introducing the containers through the cargo hatch and for turning them through 90.degree., in accordance with the state of the art the floor of the freight compartment was provided for example in the region of the cargo hatch with a ball-type conveyor track which permits the container to be moved in all directions and turned. The ball-type conveyor track includes a large number of drive rollers which can be raised and lowered between a rest position and an operative position and which can be driven in rotation in the operative position. Suitable control by means of complicated electronic control assemblies meant that such an arrangement could be used to pass the container into the cargo hatch and at the same time turned through 90.degree. during that movement. That operation was effected by the individual drive roller units being selectively raised and lowered; in the lowered condition, they performed a rotary movement about a vertical axis while in the raised condition they moved the container forwardly in its turned position.
In practice such an arrangement is not only found to be complicated and expensive but in addition it suffers from the disadvantage of being of high weight, which is undesirable in the context of air freight traffic. It is also found to be susceptible to trouble.
Another procedure employed in particular for containers of smaller size involves manually moving the containers on the ball-type conveyor track which is disposed in the region of the cargo hatch, and manually turning same. Instead of ball-type conveyor tracks, flat conveyor arrangements with directable guide rollers may also be employed.
In all the above-mentioned possible constructions, either a considerable level of expenditure is required or it is necessary to use human labour.
As mentioned above, a drive roller unit with two drive rollers is described in above-indicated U.S. patent application Ser. No. 377,971.
That has two drive rollers which are coaxially arranged in juxtaposed relationship and which are driven by an electric motor means and which are coupled together by way of a differential transmission. The differential idler pinions of the differential transmission are subjected to a braking moment which is so selected that in a first operating condition of the drive roller unit, there is a quasi rigid coupling between the two outputs of the differential transmission, which provides that the two drive rollers rotate in the same direction and at the same speed. In that first operating condition they practically act like a single drive roller, as described in DE-Al-37 24 126. The drive roller unit of the above European patent application remains in that operating condition as long as the reaction forces which are applied to the two drive rollers by the article to be driven do not exceed a magnitude which is predetermined by the braking moments at the differential idler pinions of the differential transmission. Those braking moments are matched to the mass of the articles to be driven and to the resistances to be overcome upon free movement thereof on the conveyor track, in such a way that the drive roller unit remains in the first operating condition referred to above as long as the article to be driven is not subjected to a very severe braking effect by an external force, or blocked in regard to movement in its previous direction of movement. That braking effect automatically occurs for example when the container to be loaded has been moved into the freight compartment through the side cargo hatch to such an extent that, at the side of the compartment in opposite relationship to the side with the cargo hatch, the freight container comes to bear against the side wall of the aircraft or a guide arrangement disposed at that location, and therefore cannot move any further in the previous direction, transversely with respect to the freight compartment.
The braking moments produced at the differential idler pinions in the differential transmission are overcome by the strong braking effect which is applied to the drive roller in that situation, and the previously virtually rigid coupling between the two drive rollers is eliminated. The drive roller unit now automatically changes over into a second operating condition in which the two drive rollers begin to rotate in opposite directions, as occurs at the outputs of a differential transmission, in known manner, when one of the two outputs is locked or subjected to a very strong braking effect.
Thus, in that second operating condition, one of the drive rollers continues to rotate in its previous direction of rotation while the other drive roller begins to stop or rotate backwards. Due to those relatively opposite rotary movements of the drive rollers which continue to be pressed against the underside of the article or conveyor to be moved, a turning moment about a vertical axis is produced; by virtue of the drive roller unit being mounted rotatably about a vertical axis, the turning moment produced results in a corresponding rotary movement which moves the drive roller unit into a fresh angular position in relation to the longitudinal axis of the freight compartment. That rotary movement continues until the drive roller unit occupies a position in which the article can move further along within the freight compartment at least over a certain distance therein without being impeded or while being subjected to a lowerbraking effect; the article thus moves in the direction of forward movement which is now defined by the fresh angular position of the drive roller unit, during which the article is simultaneously rotated somewhat into the new drive direction of the drive roller unit, about the vertical axis. As in that fresh position, the forces which subject the article to a substantial braking effect from outside or which totally prevent the article from moving any further, disappear or are at least reduced, the article applies lower reaction forces to the drive roller unit so that the drive roller unit automatically goes back into its first operating condition in which the two drive rollers rotate in the same forward direction. In that situation, the article to be driven will generally not yet have been turned entirely through 90.degree.. On the contrary, the above-described procedure is repeated, with the article again encountering the freight compartment wall or a corresponding abutment so that the drive roller unit is again caused to pass into the second operating condition and as a result is further rotated about the vertical axis.
That change between the two operating conditions may take place on a number of occasions until the article to be moved is finally turned through a sufficient angle, for example 90.degree., and can then be unimpededly moved in that fresh position in the longitudinal direction of the freight compartment until it reaches the intended anchorage location.
Therefore the rotary movement of the above-described twin-roller unit is produced automatically by the interplay of the mutually adapted braking moments of the article to be conveyed and the differential pinions of the differential transmission, without any necessity for intervention from the exterior, for example by means of an electronic control arrangement. It will be noted however that that twin-roller unit is of a very expensive construction, by virtue of the differential transmission required, and involves a large number of components, so that not only are its manufacturing costs high, but it is also of a high weight and suffers from a certain susceptibility to trouble.